Polymer electrolyte lithium battery containing a potassium salt

ABSTRACT

In a rechargeable lithium battery including inter alia a lithium anode, a lithium ion reducible cathode bonded with a polymer, as well as a polymer electrolyte, potassium ions are introduced either in the cathode or in the electrolyte, or in both of them at the same time, so that potassium is distributed in the cathode and the electrolyte when the generator has reached equilibrium. This has the effect of stabilizing the performances of the battery during cycling in terms of energy and power.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. application Ser. No.08/815,604 filed Mar. 12, 1997 now U.S. Pat. No. 5,798,191.

BACKGROUND OF INVENTION

a) Field of the Invention

The present invention concerns polymer electrolyte batteries, forexample, polymer electrolyte generators having a potassium salt enablingto stabilize the performances and service life of the battery. Morespecifically, the invention relates to rechargeable lithium generatorscontaining a potassium salt which is distributed in the cathode, in thepolymer electrolyte, or both of these at the same time. In particular,the present invention is directed to rechargeable electrochemicalgenerators in which the potassium ions introduced in the form ofadditives in the cathodes as well as in the polymer electrolyte definean in situ treatment which lasts the entire service life of thegenerator so as to improve performances during cycling, for example, interms of energy and power. The present invention also concerns potassiumbase additives distributed in at least one, and even two, of thecomponents of a rechargeable lithium electrochemical generator,preferably the polymer electrolyte and the composite cathode bound to apolymer, in which the effect is to stabilize the energy and powerperformances during cycling.

The invention also describes preferred means for introducing potassiuminto the generator by either one of its components and describes how thepotassium is distributed in more than one component so as to optimizethe operation of the electrodes during cycling. The additive has thebeneficial effect of maintaining the morphology of the lithium anodeduring cycling and to optimize the physical properties of the cathodeduring cycling.

b) Description of Prior Art

The life of a battery is dependent on many factors including thereversibility of the electrochemical processes at both electrodes. Theaddition of alkali earth or transition metals to the active material ofthe cathodes of lithium batteries is known and is used generally tostabilize or optimize the insertion cathodes (FR 2,616,013; U.S. Pat.No. 5,013,620; U.S. Pat. No. 5,114,809; FR 2,573,250). The additivesused are generally intended to stabilize the insertion structures andsometimes to optimize the number of sites available in the hoststructure (WO 91/02383; U.S. Pat. No. 4,668,594). In some cases, theadditives are also intended to increase the electronic conductivity ofthe insertion materials (U.S. Pat. No. 4,965,151; JP 89/15317; JP89/67063; U.S. Pat. No. 5,114,811; U.S. Pat. No. 5,147,737). In most ofthe cases known, the addition metals are integrated in the hoststructure and are present at relatively high rates which vary between 1%and 50% with respect to the main transition metal. These additives aregenerally immobilized in the host insertion structure and are notdiffused in the other components of the generator, for example, theelectrolyte and the anode. In the case where the addition metals wouldbe soluble in the electrolyte, they would be reduced with metalliclithium and could not remain in equilibrium in the generator. Moreover,in Applicant's view, no example of additive which is present in morethan one component of a lithium generator has been described up to now.

As a matter of fact, a few of these metals are chemically compatiblewith a lithium anode and are capable of coexisting with the lithiumsalts which are in solution in the electrolyte of the generator.Potassium, with magnesium is one of the only metals which are notreduced (thermodynamically and kinetically) by lithium in aprotic mediaand therefore constitutes a unique material to carry out the presentinvention.

The utilization of polymer electrolytes with mixed alkali cation hasbeen mentioned during conductivity measurements (ACFAS 1993) and for theconstitution of vehicular conduction electrolytes (cf. U.S. Pat. No.5,350,646). None of these cases mention an equilibrium between the mixedcations of the electrolyte and the materials of the electrode or abeneficial effect on the cycling of the generator or on the lithiumanode.

It is an object of the present invention to provide for a beneficialeffect noted on the stabilization of the material of the cathode V₂ O₅which is presently used in the technology of polymer electrolytebatteries and simultaneously to provide an improvement to thereversibility of the dissolution-redeposition of the lithium anode andits morphology in order to improve the performances and service life ofthe battery.

SUMMARY OF INVENTION

It is therefore an object of the invention to provide improvements bythe introduction of a potassium salt, such as KTFSI (potassiumtrifluoromethanesulfonyl imide) in the polymer separator and/or in thepolymer which constitutes the composite positive or cathode.

It is another object of the invention to introduce potassium ions in thepolymer, which are subsequently present in the electrodes byelectrochemical means.

It is also an object of the invention to provide for the manufacture ofa battery characterized by an improved cycling with respect to energyand power obtained by in situ addition of a stabilizing agent, such aspotassium, without necessarily utilizing a chemical means.

It is another object of the invention to permit the introduction of astabilizing agent, such as in the polymer electrolyte separator and/orin the cathode, so that it is uniformly distributed in the entirebattery from the interface Li to the collector of the positive.

It is also an object of the invention to produce depolarizing effectsduring recharge (decrease of voltage of more than 20 mV in the case ofdendrite formation, contact of the anode with the cathode) following themorphological development of lithium (modification of a plane surface oflithium into a rugged surface), which enables the battery to cycle againin a reversible manner for many tens of cycles without any appearance ofdendrites.

In order to achieve these objects and to overcome the disadvantages ofthe prior art, the invention proposes a rechargeable lithium batteryincluding at least one lithium anode, one lithium ion reducible cathodebound with a first polymer, and a polymer electrolyte comprising asecond polymer, and a lithium salt in solution in the second polymer.The lithium generator according to the invention is characterized inthat it contains potassium ions, which are distributed in at least oneamong the cathode and the polymer electrolyte, the concentration oflithium and potassium under equilibrium in the second polymer expressedas O/(Li+K), being between about 8/1 and 40/1, with a Li/K molar ratiobetween about 0.2 and 15. The potassium ions are selected so as tostabilize the energy and power performances of the generator duringcycling.

In general, the potassium ions are introduced by means of potassiumsalts. The potassium salt may be distributed in the cathode, in theelectrolyte, or both in the polymer electrolyte and the cathode.

Moreover, the first and second polymers may be identical or differentdepending on circumstances, as this will appear to one skilled in theart.

Examples of potassium salts which may be used in accordance with theinvention include KBF₄, KPF₆, KN(R_(F) SO₂)₂, KN(FSO₂)₂, K(FSO₂₋₋ N--SO₂R_(F)) and KR_(F) SO₃, wherein R_(F) represents a perhalogenoalkyl,perhalogenooxyalkyl, perhalogenothiaalkyl or perhalogenoaryl groupoptionally containing aza or oxa substituents and preferably having 1 to10 carbon atoms. In the salt of the formula KN(R_(F) SO₂)₂, the twoR_(F) groups may be identical or different and may form together aperhalogenated ring having 1 to 6 carbon atoms and optionally containingin the ring one or more oxygen or nitrogen atoms. The potassium ions canbe incorporated in the cathode which is in an oxidized or partially orcompletely reduced form.

Preferably, the cathode includes at least one compound selected fromoxides, sulfides or chalcogenides of transition metals, for example,vanadium pentoxide.

The compound which constitutes the cathode may be selected among thoserepresented by the formula:

    [--R--S.sub.x ].sub.n

in which R is a di- or tri-radical. Examples of di-radicals include forexample sulfur, alkylene groups containing 2 to 10 carbon atoms,oxyalkylene groups containing 4 to 12 carbon atoms, and 1 to 4 oxygenatoms, cyclic radicals such as substituted or unsubstituted phenylene,thiadiazodi-yl and oxadiazodi-yl. An example of tri-radical includes aderivative of 1,3,5 triazine on which three sulfur atoms aresubstituted, n is the degree of polymerization, which is comprisedbetween 2 and 100,000, preferably between 10 and 10,000, and X is ≧2,the potassium then being present in the cathode in the form of R--S--Kwhere R is such as defined above. A mixture of polymers comprising atleast one electronically conductive polymer and a compound of the type[--R--S_(x) ]_(n) may also constitute the cathode. It may also beselected among those represented by the formula:

    MX.sub.z

in which M is a transition metal, X represents a chalcogen or a halogen,and z varies between 1 and 3, potassium then being present in thecathode in the form of KX, where X is such as defined above.

The invention also concerns a lithium ion reducible cathode, forrechargeable lithium generator, bound to a polymer, potassium ions beingdistributed in the cathode in such quantity that the concentration oflithium and potassium in a polymer electrolyte of a generator made ofcathode when the generator has reached equilibrium, expressed asO/(Li+K) varies between about 8/1 and 40/1 while the molar ratio Li/K isbetween about 0.2 and 15.

The invention also concerns a polymer electrolyte for a rechargeablelithium battery, potassium ions being distributed in the electrolyte insuch quantity that the concentration of lithium and potassium in thepolymer electrolyte of a generator including the latter when thegenerator has reached equilibrium, expressed in O/(Li+K) varies betweenabout 8/1 and 40/1 while the molar ratio Li/K is between about 0.2 and15.

The amount of potassium salt, such as KTFSI with respect to the lithiumsalt currently used in a polymer electrolyte generator, varies to alarge extent and is preferably within a K/Li ratio lower than 5,preferably between 0.2 and 1.

When the cathode contains V₂ O₅, the maximum concentrations of K.sub.αV₂O₅, are preferably those where α≦0.06 or still where K/V is lower thanor equal to 0.03.

BRIEF DESCRIPTION OF DRAWINGS

Other characteristics and advantages of the present invention willappear from the annexed drawings in which:

FIG. 1 is a graph representing comparative results of a batteryaccording to the invention with potassium ions and another battery ofthe prior art without potassium;

FIG. 2 is a graph comparing the performances of three generatorsaccording to the invention whose composition in Li/K is variable;

FIG. 3 represents a chemical analysis by (X-ray fluorescence) EDX of theelectrolyte in cryogenic cross-section for a battery O/M=40; Li/K-0.8,after 200 cycles;

FIG. 4 represents the same analysis as FIG. 3, except that it isconcerned with the positive electrode;

FIG. 5 represents a chemical analysis by EDX of the electrolyte of thesame battery as the one illustrated in FIGS. 3 and 4 before cycling,demonstrating a higher concentration of potassium salt (ratio K/S) thanafter cycling (FIG. 3);

FIG. 6 is a micrograph obtained by microscopic electronic transmission(MET) which verifies the presence of potassium in the structure ofvanadium oxide;

FIG. 7 represents a micrographic analysis by transmission electronicmicroscopy according to the corresponding EDX spectrum obtained at theheart of a grain of vanadium oxide establishing the presence ofpotassium.

FIG. 8 represents a chemical analysis by EDX of the surface of lithiumin contact with the electrolyte at the interface Li°/polymerelectrolyte;

FIG. 9 is a cryogenic cross-section of a battery Li°-polymerelectrolyte/polymer electrolyte-Li° with KTFSI in the electrolyte;

FIG. 10 is a cryogenic cross-section similar to that of FIG. 9, withoutKTFSI;

FIGS. 11 and 12 are comparative views (75°) with a scanning electronicmicroscope (SEM) of the surface of lithium at the interface Li°/SPE ofbatteries 6 and 7 which are mentioned in FIGS. 9 and 10;

FIGS. 13 and 14 are cryogenic cross-sections of batteries 1 and 3 whichare mentioned in FIGS. 1 and 2;

FIG. 15 illustrates the evolution of the maximum value of theinstantaneous power P_(i) as a function of cycling for batteries 1 and 2which are mentioned in FIG. 1;

FIG. 16 illustrates curves comparing the physical properties ofbatteries 9 and 10 without and with K with respect to the powersustained therein;

FIG. 17 is an EDX spectrum of the electrolyte of battery 11 showing thepresence of K (originally inserted by chemical means in V₂ O₅) underequilibrium between the cathode and the polymer electrolyte; and

FIG. 18 illustrates the evolution of the specific impedance per unitarea (ASI) according to a procedure called "Dynamic Stress Test" as afunction of the number of cycles, obtained with a battery having K⁺ ionsincorporated in situ into the polymeric binder of the cathode, the K⁺ions being introduced in the form of the salt KN(FSO₂)₂.

DESCRIPTION OF PREFERRED EMBODIMENT

The addition of potassium, for example, in the form of KN(CF₃ SO₂)₂ inthe electrolyte enables to maintain a rate of use of the active materialof the cathode higher than what has been obtained in the case of asimilar generator containing no potassium. It is also shown by means ofelementary analysis after cycling that potassium which is introducedthrough the electrolyte is equally distributed in all the components ofthe battery without however being deposited at the anode which confirmsthe stability of the electrolytes with mixed alkali cations (Li⁺ +K⁺)used in the generators of the invention.

Another benefit of the present invention concerns the quality of thecontact between metallic lithium and the polymer electrolyte which ismaintained during discharge/charge cyclings which largely contributes toexcellent properties of cycling and power of the generators according tothe invention.

The possibility of introducing potassium in equilibrium in more than onecomponent of the generator by means of the material of the cathode isalso within the spirit of the invention. In these cases, the addition ofpotassium may be carried out by chemically pre-inserting potassium inthe structure of vanadium oxide by means of a solution of an oxidizablesalt such as KI in an aprotic solvent such as acetonitile so as to giveK_(x) V₂ O₅ and iodine (or triiodide). It will also be shown byelementary analysis that in these cases potassium is also present in theelectrolyte of the separator after cycling and analysis of the battery.

EXAMPLES

The examples which follow are given only as illustration and withoutlimiting the scope of the invention.

Example 1

In this example a comparison is made of the results obtained duringcycling (FIG. 1) of a battery having K⁺ incorporated in situ in thepolymeric separator film (battery 1) with a battery having no K⁺ butonly Li⁺ (battery 2). K⁺ is introduced into the battery in the form of asalt of K designated bis-trifluoromethane sulfoneimide (KTFSI)completely compatible with a polymer electrolyte battery having alithium metal anode. Batteries 1 and 2 are made of a lithium anode 35 μmthick supported on a thin sheet of nickel and a composite cathode of acomposition by volume of about 40% vanadium oxide, 10% acetylene blackand 50% of an ethylene oxide copolymer. This copolymer includes about80% ethylene oxide as described in the following patents: EPO 0,013,199;U.S. Pat. No. 4,578,326; and U.S. Pat. No. 4,758,483, to which there isadded the electrolyte bistrifluoromethane sulfoneimide of lithium(LiTFSI) and/or of potassium (KTFSI) in an oxygen:ionic metal ratio O/Mof 30/1. The cathode of a true capacity near 4C/cm² (2 Li/V) is placedon a thin nickel collector. The thickness of the separator or of thepolymer electrolyte is 30 μm and the latter is also made of an ethyleneoxide base polymer. The ratio Li/K is equal to 0.8 in the case ofbattery 1. The batteries with useful surface of 3.89 cm² are assembledby hot pressing at 80° C.

The batteries are cycled at an imposed current density of the order of100 μA/cm² in discharge and 50 μA/cm² in charge at 60° C. between limitsof 3.3V and 1.5V, thus enabling to produce deep discharges (100% DOD).FIG. 1 illustrates the evolution of the cycled capacity, or morespecifically, the percent of utilization (cycle capacity at cycle n overthe capacity obtained at cycle 1) as a function of the number of cycles.The initial rate of utilization of battery 1 is smaller than battery 2.On the other hand, the decrease of the capacity of battery 1 isstabilized after about 100 cycles at a much lower rate (6 to 7 times)than battery 2. The stabilization effect due to the presence of K in thebattery is therefore perceived beyond 100 cycles and thus permits anexpectation of service life of more than 1000 cycles for the generator.This stabilization becomes very interesting particularly towards 375cycles where there is a crossing of the two curves of the loss ofcapacity as a function of the number of cycles.

Example 2

In this Example a comparison is made between the performances of threegenerators in which the Li/K composition varies. KTFSI is introducedinto the polymer electrolyte of the separator. The batteries areessentially composed of the same anode and cathode as in Example 1. Themethod of assembly is also identical as well as the current densitiesimposed in discharge and in charge under the same limits of voltage. Allthe batteries are composed of a total quantity of salt O/M=30/1. TheLi/K ratio in the generator is equal to 0.8 in the case of battery 1, 7for battery 3 and 25 for battery 4. FIG. 2 shows the evolution of thepercentage of utilization of these three batteries as a function of thenumber of cycles. The evolution of battery 3 in terms of cycling andinternal resistance is the same as battery 1. Indeed, for this type ofelectrode material a stabilization of the slope of decrease is alsonoted after 100 cycles and is completely similar to that of battery 1when the internal resistance is stabilized between the 100th and 350thcycle for each battery, the internal resistance of battery 3 beingslightly superior. The slope of decrease of battery 4 is much higher(about 4 times) than that of battery 3 or battery 1 and of the sameorder of importance as battery 2. The concentration of K in thegenerator is therefore too low to have a beneficial effect as astabilizing additive. This result enables to conclude that a ratio whichis lower than or equal to 25 in Li/K, but higher than or equal to 7 inLi/K, is sufficient to stabilize the loss of capacity during cycling ofthe generator. The maximum concentration in K may be Li/K=0.2 with aO/M=8 to respect the electrochemical compatibility of the generator withits lithium anode.

Example 3

In this Example the intention is to illustrate that the potassiumadditive is dispersed in a homogeneous manner in the entire generator,whether it be introduced into the separator electrolyte and/or in thecathode. The present Example (battery 5) illustrates the case whereKTFSI is introduced into the separator. The quantity of salt O/M isequal to 40/1 while the ratio Li/K in the generator is equal to 0.8. Thethickness of the separator is 40 microns while the other components ofthe generator are identical to battery 1. After 200 cycles this batterywas examined by X-ray fluorescence (EDX) following a cryogenic fractureenabling to have a cross-section view of the battery.

The relative composition in "K⁺ " in the separator electrolyte and inthe positive electrode is identical, as demonstrated in FIGS. 3 and 4 bythe ratios of the intensities of the peaks representing K and S. Thesource of S in the generator comes from the anion TFSI of the salts ofLi and K. It was also noted that the ratio K/S was higher in theelectrolyte before cycling (FIG. 5), which clearly demonstrates that K⁺is redistributed in the entire generator and that there is anequilibrium in the generator for at least two components.

The presence of K in the structure of vanadium oxide (FIG. 6) was alsoverified by transmission electronic microscopy (TEM) also clarifyingwithout any doubt the presence of K (under equilibrium) at the cathodewith respect to the solid particles and the polymer electrolyte actingas binder. On the other hand, the quantity of K which is introduced intothe structure is low with respect to the concentration in the binder(FIG. 7) and therefore does not disturb, as a function of theutilization of the generator, the equilibrium between K⁺ and Li⁺ in thepolymer electrolyte.

This equilibrium probably exists at the level of the interfaceLi/polymer electrolyte. As a matter of fact, FIG. 8 clearly shows theabsence of reduced potassium (K) at the surface of metallic lithium(thus confirming the apparent stability of the K ions in the presence ofthe lithium anode in a polymer electrolyte medium, thus justifying thatK⁺ is completely compatible with the anode). Lithium is not detected bythis technique, however, oxygen (O) which covers it is clearly visibleon the spectrum. It is therefore probable that K⁺ is equally inequilibrium very close to the surface of the lithium since there is noreduction of potassium on the anode.

Thus, through these analyses it is established that K is present in morethan two components and/or sections of the battery: the polymerelectrolyte separator, the binder of the cathode which is made of thesame electrolyte as the separator, and the granular particle of theoxide of the positive electrode. The probability of finding ionicpotassium at the interface Li/polymer is also not excluded.

Example 4

In order to establish once again the beneficial effect of the presenceof K⁺ at the lithium anode, two symmetrical batteries (6 and 7) havinglithium anodes and cathodes have been assembled. FIGS. 9 and 10illustrate once again a cryogenic cross-section view with a scanningelectronic microscope (SEM) of these two batteries while FIGS. 11 and 12illustrate a view of the surface. Battery 7 was assembled with metalliclithium initially 35 μm supported on a sheet of nickel 10 μm. Thethickness of the electrolyte was about 30 μm. The electrolyte containedonly LiTFSI at a concentration of 30/1. Battery 6 is similar to battery7 except for two differences: It is made of Li with a thickness of 22 μmand it contains a quantity of salt O/M=20/1 at a ratio Li/K=2. Anotherbattery (battery 8 not illustrated) was also experimented and containeda quantity salt O/M=25 at a ratio of Li/K=5. Each of these batteries wascycled at current densities and under experimental conditions similar tothose described in Example 1. The anode was an electrode of lithiumwhich is oxidized under a double current with respect to its reductionand a second electrode was used as cathode where opposite currents werenoted. The times of cycling are adjusted at the same quantity ofcoulombs are discharged and recharged. Battery 7 has undergone 24 cyclesbefore showing internal short-circuits while battery 6 was voluntarilystopped after 39 cycles and had not yet shown major dendrites as in thecase of battery 7. The lithium noted in FIGS. 9 and 12 are those whichhave been cycled as anode, which means a deposition or a plating at acurrent density of the order of 50 μA/cm².

The developed morphology of the lithium anode of battery 7 is threetimes that of battery 6 even for a lower duration of cycling (nearlyless than half the number of cycles). The only major difference betweenthese two batteries is the presence of KTFSI in battery 6. Thus, thesepictures establish very clearly the beneficial effect of K on theprofilometry of cycled lithium and thus on the duration of the life ofthe battery. The front face microscopic views are also quite revealing.Similar conclusions have been realized following an examination ofbattery 8 which has achieved 20 cycles.

Example 5

In this Example a post mortem analysis of batteries 1 and 3 was made(cryogenic cross-section view) to illustrate (FIGS. 13 and 14) the finalstate of the morphology of different components of the battery as seenwith a scanning electronic microscope (SEM). It is noted that battery 1achieved 1100 cycles and battery 3 near 600 cycles. As will be noted,the films of polymer electrolyte and of the cathode are still quiteapart and it will be noted that the microscopic morphology of lithium isnearly nonexisting as seen from the surface of the polymer electrolyte(lithium being delaminated from the polymer electrolyte, one has toconclude that Li has a low morphologic development). As a matter offact, previous experiments have established that a view of the surfaceof the polymer electrolyte at the interface of the anode represented themirror image of the surface of lithium. The inhibiting effect ofpotassium anode on the development of the morphology of lithium is alsodemonstrated from these results.

Example 6

In this Example the beneficial effect of the additive K⁺ on theevolution of the instantaneous power as a function of the life of thegenerator is confirmed. The two batteries which were investigated areessentially the same as those of Example 1. The instantaneous power(P_(i)) is determined when the generator is fully charged. Currentdensities (I) of the order of 1 to 5 mA/cm² are provided on the batteryfor 20 seconds. Between each call for power the battery is allowed torest for 120 seconds. The final voltage (V) of each impulsion is thenregistered and the instantaneous power (mW) is given by the equationP_(i) =VI. FIG. 15 illustrates the evolution of the maximum value ofP_(i) as a function of cycling. It can be observed that the power ofbattery 1 stabilized between 200 and 600 cycles, which is not the casefor battery 2. Similarly during cycling, battery 2 has a higherinstantaneous power during the first 200 cycles but is never stabilized.The presence of a quantity of K in the ratio Li/K=0.1 is therefore verybeneficial for the stabilization of the instantaneous power. Theinternal resistances of the batteries are also indicated in FIG. 15. Theinternal resistance of battery 1 is slightly superior to that of battery2 for the first 100 cycles while at cycle 300 the value of the internalresistance of battery 2 is higher. This difference may be the reason forbetter performances in power of battery 1.

Example 7

In this Example, the physical properties of batteries with and without Kwith respect to their sustained power (Ragone curve for configurationsof optimized batteries for metallic collectors) are compared. Battery 9(FIG. 16) contains no K and is of the same nature as battery 2 mentionedin Example 1. Battery 10 is exactly identical to battery 3 cited inExample 2. The quantity of K is introduced into the polymerelectrolyteby means of compound KTFSI at a rate to give a concentrationLi/K=7(O/M=30). The assembly and cycling of the batteries are identicalto the pervious Examples.

As already mentioned, the lower power energy of battery 9 is higher thanthat of battery 10 since its rate of utilization (up to about 375cycles) is higher. The initial power is also higher than the batteryhaving K. On the other hand, after 200 cycles, battery 9 without K showsa considerably reduced specific energy (wh/kg) especially under highpower of the order of 200 W/kg, which is not the case for battery 10. Asa matter of fact, although it is lower at the start of the service lifeof the generator, the sustained power of the battery having K ismaintained, and this for more than 300 cycles, which demonstrates thestabilizing effect brought about by K in the polymer electrolyte lithiumbattery.

Example 8

In this Example, it is shown that the equilibrium of the species Li andK may be obtained in at least three components including vanadium oxide,the polymer which binds the cathode, and the polymer of the electrolyte,due to the chemical addition of potassium in the structure of vanadiumoxide by means of a solution of KI in acetonitrile so as to give K_(x)V₂ O₅ and iodine (or triiodide). After a cycling of the same type asdescribed in Example 1, battery 11 was examined by X-ray fluorescence(EDX) following a cryogenic fracture enabling to obtain a cross-sectionview of the battery. The electrochemical configuration of battery 11 isidentical to that of battery 1 except that the electrolyte contains nopotassium salt and that vanadium oxide V₂ O₅ contains about 0.18 mole ofK. The spectrum EDX is illustrated in FIG. 17. The presence of K in thepolymer electrolyte and/or the cathode may be observed while nopotassium has been introduced into the starting electrolyte of thepolymer binding the cathode.

Example 9

In this example, a comparison is made between the results obtainedduring cycling of a battery having K⁺ ions incorporated in situ into thepolymeric binder of the cathode (battery 12) and the results obtainedwith a battery having no K⁺ ions but only Li⁺ ions (battery 13). The K⁺ions are introduced into the cathode in the form of potassiumbis-fluoromethane sulfoninide having the formula KN(FSO₂)₂ hereinafterreferred to as KFSI. This salt is completely compatible with a polymerelectrolyte battery having a lithium metal anode. Batteries 12 and 13are each made of an anode consisting of a self-supported lithium filmhaving a thickness of 27 μm, and a cathode consisting of a compositematerial having a thickness of 15 μm and comprising 40 vol. % vanadiumoxide, 10 vol. % acetylene black and 50 vol. % of an ethylene oxidecopolymer similar to the one described in Example 1. In battery 12, thecopolymer contains a mixture of KFSI and lithium bis-trifluoromethanesulfonimide (LiTFSI) in an oxygen: ionic metal ration O/(Li+K) of 30/1,the Li/K ratio being equal to 2. In battery 13, the copolymer containsthe salt LiTFSI in an oxygen: ionic metal ratio O/Li of 30/1. In bothbatteries, the cathode which has a useful capacity of about 5 C/cm² isplaced on a metal collector. The separator between the anode and thecathode comprises an ethylene oxide copolymer film having a thickness of15 μm. The batteries are assembled by hot pressing at 80° C. the filmsconstituting the anode, cathode and separator, and they have a usefulsurface of 3.89 cm².

The batteries are cycled according to a procedure called "Dynamic StressTest" (DST) comprising several discharging and charging steps(regenerative breaking in a configuration reproducing the operation ofan electric vehicle leading to a complete discharge of the battery (thedepth of discharge DOD being 80%). The charging rate of the batteries isC/10, corresponding to a complete charging in 10 hours.

FIG. 18 illustrates the evolution of the specific impedance per unitarea (ASI) according to the DST profile as a function of the number ofcycles. The DST ASI is determined at a depth of discharge of 80% andrepresents the resistance of the cell. The DST ASI is evaluated during apeak discharge of 8 seconds followed by a peak charge of 8 seconds(regenerative breaking). FIG. 18 shows that the presence of K₊ ions iscompletely compatible with other materials in the cell and does notaffect the maximum power of the cell. The partial substitution of LiTFSIwith a less costly salt such as KN(FSO₂)₂ without a lowering of thebattery performances proves to be very interesting for the developmentof electrochemical generators.

It is understood that the invention is not restricted to the Examplesgiven above, and that modifications and alternatives are possiblewithout departing from the scope of the invention.

We claim:
 1. Rechargeable lithium battery comprising at least onelithium anode, one lithium ion reducible cathode bonded to a firstpolymer, and a polymer electrolyte comprising a lithium salt in solutionin a second polymer, said lithium battery containing an additivecomprising potassium ions introduced in the form of potassium saltsselected from the group consisting of KN(R_(F) SO₂)₂, KN(FSO₂)₂, K(FSO₂--N--SO₂ R_(F)) and KR_(F) SO₃ in which R_(F) represents aperhalogenoalkyl, perhalogenooxyalkyl, perhalogenothiaalkyl orperhalogenoaryl group optionally containing aza or oxa substituents, andthe two R_(F) groups in the KN(R_(F) SO₂)₂ are identical or different orform together a perhalogenated ring having 1 to 6 carbon atoms andoptionally containing in the ring at least one oxygen or nitrogen atom,said potassium ions being distributed in at least one of said cathodeand said polymer electrolyte, the concentration of lithium and potassiumin the second polymer when said battery has reached equilibrium,expressed as O/(Li+K), being between about 8/1 and 40/1 while the molarratio Li/K is between about 0.2 to 15, said potassium ions beingselected so as to stabilize performances of the battery during cyclingin terms of energy and power.
 2. Rechargeable lithium battery accordingto claim 1, wherein said potassium salt is distributed in said cathode.3. Rechargeable lithium battery according to claim 1, wherein saidpotassium salt is introduced in said polymer electrolyte. 4.Rechargeable lithium battery according to claim 1, wherein saidpotassium salt is distributed in said polymer electrolyte and saidcathode.
 5. Rechargeable lithium battery according to claim 1, whereinsaid first and said second polymers are similar.
 6. Rechargeable lithiumbattery according to claim 1, wherein said first and said secondpolymers are different.
 7. Rechargeable lithium battery according toclaim 1, wherein said potassium salt is selected from the groupconsisting of K(FSO₂ --N--SO₂ R_(F)) and KR_(F) SO₃, in which R_(F)represents a perhalogenoalkyl, perhalogenooxyalkyl, perhalogenothiaalkylor perhalogenoaryl group having 1 to 10 carbon atoms.
 8. Rechargeablelithium battery according to claim 1, wherein said potassium ions areincorporated by means of said cathode, said cathode being in completelyor partly reduced form.
 9. Rechargeable lithium battery according toclaim 2, wherein said cathode includes at least one compound selectedfrom the group consisting of oxides and chalcogenides of transitionmetals.
 10. Rechargeable lithium battery according to claim 9, whereinsaid compound consists of V₂ O₅.
 11. Rechargeable lithium batteryaccording to claim 9, wherein said compound is represented by theformula [--R--S_(x) ]_(n), wherein R is a di- or tri-radical, n is awhole number between 2 and 100,000, and X>2, potassium in said cathodeis then in the form of R--S--K, wherein R is as defined above. 12.Rechargeable lithium battery according to claim 11, wherein saidcompound is represented by the formula MX_(z) wherein M is a transitionmetal, X is a chalcogen or a halogen, and Z varies between 1 and 3,potassium in said cathode is then in the form of KX, wherein X is asdefined above.
 13. A method of making a rechargeable lithium battery,the method comprising introducing potassium ions into a polymerelectrolyte battery; and forming the rechargeable lithium battery ofclaim
 1. 14. A method of using a rechargeable lithium battery, themethod comprising discharging of charging the rechargeable lithiumbattery of claim 1.